In Pale Moon in Windows I can use Ctrl W to close the current tab, but this doesn't work in Linux Mint Mate 19.Is this specific to Pale Moon or to Mate? Is there a Key combination I can use? Would it be different in Mint Cinnamon?

the Ctrl W shortcut is universal across all OSes, so the culprit has gotta be something specific to your distro. are you using any Unicode typing software or keyboard layout that alters the output of the W key?